1. Field of the Invention
The present invention relates to the control of brushless C motors.
2. Description of the Related Art
In control of a brushless DC motor, a controller generates appropriate signals in the stator windings of the motor to create magnetic flux which interacts with permanent magnets in the rotor. The controller typically has direct control of switching devices in an inverter, the three outputs of which (in the case of a three-phase stator) are provided to the stator windings.
In order for the controller to generate its signals in synchronism with the rotation of the rotor (and thereby maintain the "commutation" which the controller attains), knowing the speed and/or position of the rotor relative to the rotating magnetic field generated by the stator windings is important. Certainly, a sensor dedicated to sensing rotor position and/or spied will suffice. However, such a sensor adds cost and unreliability.
In another known design, a virtual "neutral" of the stator windings is created by tying the three line voltage signals of the stator together through buffer resistances. The signal at the virtual neutral is compared with the three line voltage signals. The resulting three comparison signals are useful in determining rotor position and speed However, the signals must be tracked in a computationally-intensive manner. The generation of three comparison signals and the computationally-intensive nature of tracking the signals are relatively expensive to implement.
Therefore, a way to track relative rotor speed in a more economical manner will provide advantages over the prior art.